The proposed plan is an evaluation of 125I-iodocyanopindolol (ICYP) as a possible prolactinoma imaging agent. Prolactinomas are tumors affecting about 25% of all women with amenorrhea. Specific prolactinoma imaging may, (1) permit earlier detection, (2) increase diagnostic precision, and (3) permit serial assessment of medical therapy. In preliminary studies we have evaluated a D2 receptor antagonist, spiroperidol, as an imaging agent. This previous work focused on use of the endogenous inhibition of prolactin (PRL) release as a means of imaging. The present proposal focuses on use of the endogenous stimulation of PRL release as a possible target for imaging. ICYP was selected for initial evaluation based on its high affinity for the Beta adrenergic receptor implicated in stimulation of PRL release. The proposal details several experiments designed to permit assessment of imaging potentials. These experiments include: (1) preliminary tissue distribution studies in both normal and tumor-induced rat pituitary tissue, (2) quantification of Beta adrenergic receptor numbers in both normal and tumor-induced pituitaries, (3) determination of the degree of uptake associated with the D2 receptor, and (4) competition studies using an in vitro model. Protocols for these experiments are included.